Torque Distributing Drive Mechanism For Motor Vehicles

ABSTRACT

A drive axle assembly includes first and second axleshafts and a drive mechanism coupling a driven input shaft to the axleshafts. The drive mechanism includes a differential assembly, a planetary gear assembly operably disposed between the differential assembly and the first axleshaft, a brake and first and second mode clutches. The first clutch is operable with the brake and the planetary gear assembly to increase the rotary speed of the first axleshaft which, in turn, causes a corresponding decrease in the rotary speed of the second axleshaft. The second clutch is operable with the brakes and the planetary gear assembly to decrease the rotary speed of the first axleshaft so as to cause an increase in the rotary speed of the second axleshaft. A control system controls actuation of both mode clutches.

CROSS REFERENCE

This application is a continuation of U.S. patent application Ser. No.11/396,138 filed on Mar. 31, 2006, which claims the benefit of U.S.Provisional Application Ser. No. 60/694,475 filed Jun. 28, 2005. Theentire disclosures of each of the above applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to differential assemblies foruse in motor vehicles and, more specifically, to a differential assemblyequipped with a torque vectoring drive mechanism and an active controlsystem.

BACKGROUND OF THE INVENTION

In view of consumer demand for four-wheel drive vehicles, many differentpower transfer system are currently utilized for directing motive power(“drive torque”) to all four-wheels of the vehicle. A number of currentgeneration four-wheel drive vehicles may be characterized as includingan “adaptive” power transfer system that is operable for automaticallydirecting power to the secondary driveline, without any input from thevehicle operator, when traction is lost at the primary driveline.Typically, such adaptive torque control results from variable engagementof an electrically or hydraulically operated transfer clutch based onthe operating conditions and specific vehicle dynamics detected bysensors associated with an electronic traction control system. Inconventional rear-wheel drive (RWD) vehicles, the transfer clutch istypically installed in a transfer case for automatically transferringdrive torque to the front driveline in response to slip in the reardriveline. Similarly, the transfer clutch can be installed in a powertransfer device, such as a power take-off unit (PTU) or in-line torquecoupling, when used in a front-wheel drive (FWD) vehicle fortransferring drive torque to the rear driveline in response to slip inthe front driveline. Such adaptively-controlled power transfer systemcan also be arranged to limit slip and bias the torque distributionbetween the front and rear drivelines by controlling variable engagementof a transfer clutch that is operably associated with a centerdifferential installed in the transfer case or PTU.

To further enhance the traction and stability characteristics offour-wheel drive vehicles, it is also known to equip such vehicles withbrake-based electronic stability control systems and/or tractiondistributing axle assemblies. Typically, such axle assemblies include adrive mechanism that is operable for adaptively regulating theside-to-side (i.e., left-right) torque and speed characteristics betweena pair of drive wheels. In some instances, a pair of modulatableclutches are used to provide this side-to-side control, as is disclosedin U.S. Pat. Nos. 6,378,677 and 5,699,888. According to an alternativedrive axle arrangement, U.S. Pat. No. 6,520,880 discloses ahydraulically-operated traction distribution assembly. In addition,alternative traction distributing drive axle assemblies are disclosed inU.S. Pat. Nos. 5,370,588 and 6,213,241.

As part of the ever increasing sophistication of adaptive power transfersystems, greater attention is currently being given to the yaw controland stability enhancement features that can be provided by such tractiondistributing drive axles. Accordingly, this invention is intended toaddress the need to provide design alternatives which improve upon thecurrent technology.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide adrive axle assembly for use in motor vehicles which are equipped with anadaptive yaw control system.

To achieve this objective, the drive axle assembly of the presentinvention includes first and second axleshafts connected to a pair ofwheels and a drive mechanism that is operable to transfer drive torquefrom a driven input shaft to the first and second axleshafts. The drivemechanism includes a differential assembly, a planetary gear assembly, abrake and first and second friction clutches. The planetary gearassembly is operably disposed between the differential assembly and thefirst axleshafts. The first friction clutch is operable in associationwith the brake and the planetary gear assembly to increase the rotaryspeed of the first axleshaft which, in turn, causes a correspondingdecrease in the rotary speed of the second axleshaft. In contrast, thesecond friction clutch is operable in association with the brake and theplanetary gear assembly to decrease the rotary speed of the firstaxleshaft so as to cause a corresponding increase in the rotary speed ofthe second axleshaft. Accordingly, selective control over actuation ofthe brake and one or both of the first and second friction clutchesprovides adaptive control of the speed differentiation and the torquetransferred between the first and second axleshafts. A control systemincluding and ECU and sensors are provided to control actuation of bothfriction clutches and the brake.

Pursuant to an alternative objective, the drive mechanism can beutilized in a power transfer unit, such as a transfer case, of afour-wheel drive vehicle to adaptively control the front-reardistribution of drive torque delivered from the powertrain to the frontand rear wheels.

Further objectives and advantages of the present invention will becomeapparent by reference to the following detailed description of thepreferred embodiment and the appended claims when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a diagrammatical illustration of an all-wheel drive motorvehicle equipped with a torque distributing drive axle assembly andactive yaw control system of the present invention;

FIG. 2 is a schematic illustration of the drive axle assembly shown inFIG. 1 according to a first embodiment of the present invention;

FIG. 3 is a diagrammatical illustration of the power-operated brakeactuators and the power-operated clutch actuator associated with thedrive axle assembly of the present invention;

FIG. 4 is a schematic illustration of an alternative embodiment of thedrive axle assembly of the present invention;

FIG. 5 is a diagrammatical illustration of the torque distributingdifferential assembly of the present invention installed in a powertransfer unit for use in a four-wheel vehicle; and

FIG. 6 is a schematic drawing of the power transfer unit shown in FIG.5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an all-wheel drive vehicle 10 includes an engine 12mounted in a front portion of vehicle 10, a transmission 14 driven byengine 12, a front differential 16 which connects the output oftransmission 14 to front axleshafts 18L and 18R for driving left andright front wheels 20L and 20R, a power transfer unit (“PTU”) 22 whichconnects front differential 16 to a propshaft 24 and a rear axleassembly 26 having a drive mechanism 28 which connects propshaft 24 torear axleshafts 30L and 30R for driving left and right rear wheels 32Land 32R. As will be detailed, drive mechanism 28 is operable inassociation with a yaw control system 34 for controlling thetransmission of drive torque through rear axleshafts 30L and 30R to rearwheels 32L and 32R.

In addition to an electronic control unit (ECU) 36, yaw control system34 includes a plurality of sensors for detecting various operational anddynamic characteristics of vehicle 10. For example, a front wheel speedsensor 38 is provided for determining a front wheel speed value based onrotation of propshaft 24, a pair of rear wheel speed sensors 40 areoperable to detect the individual rear wheel speed values based rotationof left and right axle shafts 30L and 30R and a steering angle sensor 42is provided to detect the steering angle of a steering wheel 44. Thesensors also include a yaw rate sensor 46 for detecting a yaw rate ofthe body portion of vehicle 10, a lateral acceleration sensor 48 fordetecting a lateral acceleration of the vehicle body, and a lock switch50 for permitting the vehicle operator to intentionally shift drivemechanism 28 into a locked mode. As will be detailed, ECU 36 controlsoperation of a pair of brakes and a torque vectoring friction clutchthat are associated with drive mechanism 28 by utilizing a controlstrategy that is based on input signals from the various sensors andlock switch 50.

Rear axle assembly 26 includes an axle housing 52 within which drivemechanism 28 is supported. In general, drive mechanism 28 includes aninput shaft 54, a differential assembly 56, a planetary gear assembly58, a first or “overdrive” mode clutch 60, a second or “underdrive” modeclutch 62 and a brake 64. As seen, input shaft 54 includes a pinion gear66 that is in constant mesh with a hypoid ring gear 68. Hypoid ring gear68 is fixed for rotation with a ring gear 70 which acts as the inputcomponent of differential assembly 56. Differential assembly 56 furtherincludes a first output component shown as a sun gear 72 that is fixedfor rotation with right axleshaft 30R, a second output component shownas a differential carrier 74 that is fixed for rotation with leftaxleshaft 30L and a plurality of meshed pairs of first pinions 76 andsecond pinions 78. Carrier 74 includes a first carrier ring 80 fixed toleft axleshaft 30L, a second carrier ring 82 and a set of first pins 84extending between carrier rings 80 and 82 on which first pinions 76 arerotatably supported. Carrier 74 also includes a set of second pins 86extending between carrier rings 80 and 82 which rotatably support secondpinions 78. As seen, first pinions 76 are meshed with sun gear 72 whilesecond pinion gears 78 are meshed with ring gear 70. While not limitedthereto, the gearing associated with differential assembly 56 ispreferably configured to normally provide an equal torque split (i.e.,50-50) to its output components.

Planetary gear assembly 58 is a ravigneaux gearset having a firstgearset 90 and a second gearset 92. First gearset 90 includes a firstsun gear 94, a first ring gear 96 and a set of first planet gears 98meshed with first sun gear 94 and first ring gear 96. Each of firstplanet gears 98 is rotatably supported on a post 100 extending betweenfirst and second carrier rings 102 and 104, respectively, that incombination define a first planet carrier 106. As seen, second carrierring 104 is fixed for common rotation with right axleshaft 30R.

Second gearset 92 includes a second sun gear 110, a second ring gear 112and a set of second planet gears 114 meshed therewith. Each of secondplanet gears 114 is rotatably supported on a post 116 extending betweenthird and fourth carrier rings 118 and 120, respectively, that incombination define a second planet carrier 122. As seen, second ringgear 112 is coupled via a first drum 124 to second carrier ring 104 offirst planet carrier 106 for common rotation with right axleshaft 30R.In addition, fourth carrier ring 120 is fixed via a second drum 126 forcommon rotation with first ring gear 96.

With continued reference to FIG. 2, drive mechanism 28 is shown tofurther include a first transfer shaft 128 that is rotatably supportedon right axleshaft 30R and which is fixed to second carrier ring 82 forcommon rotation with differential carrier 74 of differential assembly56. Likewise, a second transfer shaft 130 is rotatably supported onright axleshaft 30R and is fixed for common rotation with first sun gear94 of planetary gear assembly 58. Furthermore, a third transfer shaft132 is rotatably supported on second transfer shaft 130 and is fixed tosecond drum 126 for common rotation with first ring gear 96 and secondplanet carrier 122. As seen, first mode clutch 60 is operably disposedbetween first transfer shaft 128 and second transfer shaft 130 forselectively coupling first sun gear 94 of planetary gear assembly 58 todifferential carrier 74 which, as noted, is commonly driven with leftaxleshaft 30L. First mode clutch 60 includes a clutch drum 134 fixed forrotation with first transfer shaft 128, a clutch hub 136 fixed forrotation with second transfer shaft 130, a multi-plate clutch pack 138disposed between hub 136 and drum 134 and a power-operated clutchactuator 140.

First mode clutch 60 is operable in a first or “released” mode so as topermit unrestricted rotation of first sun gear 94 relative todifferential carrier 74. In contrast, first mode clutch 60 is operablein a second or “locked” mode for inhibiting rotation of first sun gear94 relative to differential carrier 74. First mode clutch 60 is shiftedbetween its released and locked modes via actuation of clutch actuator140 in response to control signals from ECU 36. Specifically, first modeclutch 60 is operable in its released mode when clutch actuator 140applies a predetermined minimum clutch engagement force on clutch pack138 and is further operable in its locked mode when clutch actuator 140applies a predetermined maximum clutch engagement force on clutch pack138.

Second mode clutch 62 is operably disposed between first transfer shaft128 and third transfer shaft 132 for selectively coupling first ringgear 96 of planetary gear assembly 58 to differential carrier 74 ofdifferential assembly 56. Second mode clutch 62 includes a clutch hub142 fixed for rotation with third transfer shaft 132, a multi-plateclutch pack 144 disposed between hub 142 and drum 134 and apower-operated clutch actuator 146. As seen, a reaction plate 148 drivenby clutch drum 134 is disposed between hubs 136 and 142 and separatesclutch packs 138 and 144. Second mode clutch 62 is operable in a firstor “released” mode so as to permit unrestricted rotation of first ringgear 96 relative to differential carrier 74. In contrast, second modeclutch 62 is operable in a second or “locked” mode for inhibitingrotation of first ring gear 96 relative to differential carrier 74.Second mode clutch 62 is shifted between its released and locked modesvia actuation of clutch actuator 146 in response to control signals fromECU 36. Specifically, second mode clutch 62 is operable in its releasedmode when clutch actuator 146 applies a predetermined minimum cutchengagement force on clutch pack 144 and is further operable in itslocked mode when clutch actuator 146 applies a predetermined maximumclutch engagement force on clutch pack 144.

Brake 64 is shown to be operably arranged between second sun gear 110and axle housing 52. Brake 64 includes a brake hub 150 that is fixed forrotation with second sun gear 110 and has a rim segment with a layer offriction material 152 thereon. Brake 64 also includes a power-operatedbrake actuator 154. Brake 64 is operable in a first or “released” modeto permit unrestricted rotation of second sun gear 110. With brake 64 inits released mode, brake actuator 154 is disengaged from frictionmaterial 152 on the rim segment of brake hub 148. In contrast, brake 64is also operable in a second or “locked” mode for inhibiting rotation ofsecond sun gear 110. With brake 64 in its locked mode, brake actuator154 is engaged with friction material 152 on the rim segment of brakehub 150 so as to brake rotation of second sun gear 110. Brake 64 isshifted between its released and locked modes via actuation ofpower-operated brake actuator 154 in response to control signals fromECU 36.

Power-operated clutch actuators 140 and 146 and power-operated brakeactuator 154 are shown in block format to cumulatively represent thecomponents required to accept a control signal from ECU 36 and generatean engagement force to be applied to its corresponding clutch pack orbrake hub. To this end, FIG. 3 diagrammatically illustrates the basiccomponents associated with such power-operated clutch and brakeactuators. Specifically, power-operated clutch actuators 140 and 146each include a controlled device 160 and a force generating mechanism162. In electromechanical systems, controlled device 160 would representsuch components as, for example, an electric motor or an electromagneticsolenoid assembly capable of receiving an electric control signal fromECU 36. The output of controlled device 160 would control operation offorce generating mechanism 162. Force generating mechanism 162 could becomprised of, for example, a ball ramp unit, a ball screw unit, aleadscrew unit, a pivotal lever arm, rotatable cam plates, etc., all ofwhich are capable of converting the output of controlled device 160 intoa clutch engagement force. Force generating mechanism 162 functions toalso transmit and exert the clutch engagement force onto clutch packs138 and 144 via an apply plate that is moveable into and out ofengagement with clutch packs. If a hydramechanical system is used,controlled device 160 would be a flow or pressure control valve operablein response to control signals from ECU 36 for delivering pressurizedfluid from a fluid source to a piston chamber. A piston disposed formovement in the piston chamber would act as force generating mechanism162. Preferably, controlled device 160 is capable of receiving variableelectric control signals from ECU 36 for permitting modulation of themagnitude of the clutch engagement force generated and applied to clutchpacks 138 and 144 so as to permit “adaptive” control of first and secondmode clutches 60 and 62, respectively.

Brake actuator 154 is also schematically shown in FIG. 3 to include acontrolled device 164 and a brake force generating mechanism 166.Controlled device 164 could be an electrically-operated motor orelectromagnetic solenoid capable of receiving electric control signalsfrom ECU 36. The output of controlled device 164 would control operationof force generating mechanism 166 comprised of, for example, a caliperunit capable of converting the output of controlled device 164 into abrake force that is applied to brake hub 150. Alternatively, forcegenerating mechanism 166 could be any known clamping assembly capable ofengaging and holding brake hub 150 against rotation.

In accordance with the arrangement shown, drive mechanism 28 is operablein coordination with yaw control system 34 to potentially establish fivedistinct operational modes for controlling the transfer of drive torquefrom input shaft 54 to axleshafts 30L and 30R. In particular, a firstoperational mode is established when first mode clutch 60, second modeclutch 62 and brake 64 are all in their released mode. As such,differential assembly 56 acts as an “open” differential so as to permitunrestricted speed differentiation with drive torque transmitted to eachaxleshaft 30L and 30R based on the tractive conditions at eachcorresponding rear wheel 32L and 32R. A second operational mode isestablished when first mode clutch 60, second mode clutch 62 and brake64 are all in their locked mode such that differential assembly 56 actsas a “locked” differential with no speed differentiation permittedbetween rear axleshafts 30L and 30R. This mode can be intentionallyselected via actuation of lock switch 50 when vehicle 10 is beingoperated off-road or on poor roads.

A third operational mode is established when first mode clutch 60 isshifted into its locked mode, second mode clutch 62 is shifted into itsreleased mode and brake 64 is shifted into its locked mode. As a result,first sun gear 94 of gear assembly 58 is coupled for rotation with leftaxleshaft 30L through differential carrier 74 of differential assembly56 while second sun gear 110 is braked. Upon rotation of right axleshaft30R, gear assembly 58 causes first ring gear 96 to be driven at anincreased speed relative to the rotary speed of first planet carrier106. As such, first sun gear 94 is driven by first gearset 90 at adecreased rotary speed relative to right axleshaft 30L. Therefore, leftaxleshaft 30L is underdriven relative to right axleshaft 30R due to itsconnection to first sun gear 94 via first mode clutch 60 releaseablycoupling first transfer shaft 128 to second transfer shaft 130.Accordingly, this third operational mode can be established to overdriveright axleshaft 30R when required to accommodate the current tractive orsteering conditions detected and/or anticipated by ECU 36 based on theparticular control strategy used.

A fourth operational mode is established when first mode clutch 60 isshifted into its released mode, second mode clutch 62 is shifted intoits locked mode, and brake 64 is shifted into its locked mode. As aresult, second planet carrier 122 and first ring gear 96 of gearassembly 58 are coupled to differential carrier 74 while second sun gear110 is braked. As such, second planet carrier 122 causes first ring gear96 to be driven at an increased speed relative to the rotary speed offirst planet carrier 106. Since first ring gear 96 is coupled todifferential carrier 74 via engagement of second mode clutch 62, rightaxleshaft 30R is underdriven relative to left axleshaft 30L foraccommodating the tractive or steering conditions detected and/oranticipated by ECU 36. Finally, a fifth operational mode is establishedwhen brake 64 is shifted into its released mode and engagement of firstand second modes clutches 60 and 62 is varied to provide a slip limitingfunction.

At the start of vehicle 10, power from engine 12 is transmitted to frontwheels 20L and 20R through transmission 14 and front differential 16.This drive torque is also transmitted to drive mechanism 28 through PTU22 and propshaft 24 for rotatably driving input pinion shaft 58.Typically, mode clutches 60 and 62 and brake 64 would be non-engagedsuch that drive torque is transmitted through differential unit 56 torear wheels 32L and 32R. However, upon detection of lost traction atfront wheels 20L and 20R, one or both mode clutches 60 and 62 and brake64 can be engaged to provide drive torque to rear wheels 32L and 32Rbased on the tractive needs of vehicles 10. Typically, brake 64 isoperated in its released mode, whereby planetary gear assembly 58rotates as a unit. This is beneficial since only small, if any, slipspeeds are generated across the clutch packs of mode clutches 60 and 62,thereby limiting the parasitic losses generated during most typicalstraight-ahead driving conditions. In addition, all torque transfer or“vectoring” between axleshafts 30L and 30R is transmitted throughengagement of modes clutches 60 and 62. Further, only on-off control ofbrake 64 is required since modulated control of mode clutches 60 and 62permits the adaptive torque regulation required for the torque vectoringand slip limiting characteristics.

In addition to on-off control of mode clutches 60 and 62 forestablishing the various drive modes associated with direct orunderdrive connections through planetary gearset 58, it is contemplatedthat variable clutch engagement forces can also be generated bypower-operated clutch actuators 140 and 146 to adaptively controlleft-to-right speed and torque transfer characteristics. This adaptivecontrol feature functions to provide enhanced yaw and stability controlfor vehicle 10. For example, a “reference” yaw rate can be determinedbased on the steering angle detected by steering angle sensor 42, avehicle speed calculated based on signals from the various speedsensors, and a lateral acceleration detected by lateral accelerationsensor 48 during turning of vehicle 10. ECU 36 compares this referenceyaw rate with an “actual” yaw rate detected by yaw sensor 46. Thiscomparison will determine whether vehicle 10 is in an understeer or anoversteer condition so as to permit yaw control system 34 to accuratelyadjust or accommodate for these types of steering tendencies. ECU 36 canaddress such conditions by shifting drive mechanism 28 into the specificoperative drive mode that is best suited to correct the actual oranticipated oversteer or understeer situation. Optionally, variablecontrol of mode clutches 60 and 62 also permits adaptive regulation ofthe side-to-side torque and speed characteristics if one of the distinctdrive modes is not adequate to accommodate the current steer tractivecondition.

Referring now to FIG. 4, an alternative embodiment of drive mechanism 28of FIG. 2 is shown and designated by reference numeral 28A. Generallyspeaking, a large number of components are common to both drivemechanism 28 and 28A, with such components being identified by the samereference numbers. However, drive mechanism 28A is shown to include amodified differential assembly 200 of the bevel type having adifferential carrier 202 driven by hypoid ring gear 68 that acts as itsinput component. Differential assembly 200 further includes a firstoutput component shown as a first side gear 204 that is fixed for commonrotation with right axleshaft 30R, a second output component shown as asecond side gear 206 that is fixed for common rotation with leftaxleshaft 30L and pinion gears 208 driven by carrier 202 and which areeach meshed with side gears 204 and 206. As seen, first transfer shaft128 now connects differential carrier 202 for common rotation with drum134 of friction clutches 60 and 62.

Drive mechanism 28A is generally similar in operation to drive mechanism28 except that mode clutches 60 and 62 now function to selectivelyconnect gear assembly 58 to the input component (i.e., carrier 202) ofdifferential 200 instead of directly to its second output component. Assuch, engagement of mode clutches 60 and 62 when brake 64 is engagedresults in torque vectoring between axleshafts 30L and 30R throughpinion gears 208. Thus, the rotary speed of left axleshaft 30L relativeto right axleshaft 30R can be increased or decreased by controllingon/off engagement of brake 64 and variable engagement of mode clutches60 and 62 in the same manner previously described for driven mechanism28 to establish the various distinct operational modes for drivemechanism 28A.

Referring now to FIG. 5, a four-wheel drive vehicle 10′ is shown with apower transfer unit 210 operable for transferring drive torque from theoutput of transmission 14 to a first (i.e., front) output shaft 212 anda second (i.e., rear) output shaft 214. Front output shaft 212 drives afront propshaft 216 which, in turn, drives front differential 16 fordriving front wheels 20L and 20R. Likewise, rear output shaft 214 drivesa rear propshaft 218 which, in turn, drives a rear differential 220 fordriving rear wheels 32L and 32R. Power transfer unit 210, otherwiseknown as a transfer case, includes a torque distribution mechanism 222which functions to transmit drive torque from its input shaft 224 toboth of output shafts 212 and 214 so as to bias the torque distributionratio therebetween, thereby controlling the tractive operation ofvehicle 10′. As seen, torque distribution mechanism 222 is operablyassociated with a traction control system 34′ for providing thisadaptive traction control feature.

Referring primarily to FIG. 6, torque distribution mechanism 222 ofpower transfer unit 210 is shown to be generally similar in structure todrive mechanism 28 of FIG. 2 with the exception that ring gear 70 is nowdrivingly connected to input shaft 224 via a transfer assembly 226. Inthe arrangement shown, transfer assembly 226 includes a first sprocket228 driven by input shaft 224, a second sprocket 230 driving ring gear70 and a power chain 232 meshed therebetween. As seen, front outputshaft 212 is driven by differential carrier 74 of differential unit 56which now acts as a center or “interaxle” differential for permittingspeed differentiation between the front and rear output shafts. Inaddition, sun gear 72 of differential unit 56 drives rear output shaft214 while first planet carrier 106 of gearset 90 is also coupled to rearoutput shaft 214.

Control over actuation of brake 64 and mode clutches 60 and 62 resultsin corresponding increases or decreases in the rotary speed of rearoutput shaft 214 relative to front output shaft 212, thereby controllingthe amount of drive torque transmitted therebetween. In particular, withboth of the mode clutches and the brake released, unrestricted speeddifferentiation is permitted between the output shafts while the gearratio established by the components of interaxle differential assembly56 controls the front/rear torque ratio based on the current tractiveconditions of the front and rear wheels. In contrast, with both modeclutches and the brake engaged, a locked four-wheel drive mode isestablished wherein no interaxle speed differentiation is permittedbetween the front and rear output shafts. Such a drive mode can beintentionally selected via lock switch 50 when vehicle 10′ is drivenoff-road or during severe road conditions. An adaptive four-wheel drivemode is made available under control of traction control system 34′ tovary the front-rear drive torque distribution ratio based on thetractive needs of the front and rear wheels as detected by the varioussensors by selectively engaging brake 64 and then adaptively controllingengagement of mode clutches 60 and 62. In addition to power transferunit 210, vehicle 10′ could also be equipped with a rear axle assemblyhaving either drive mechanism 28 or 28A and its corresponding yawcontrol system, as is identified by the phantom lines in FIG. 5.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A motor vehicle, comprising: a powertrain operable for generatingdrive torque; a primary driveline for transmitting drive torque fromsaid powertrain to first and second primary wheels; a secondarydriveline for selectively transmitting drive torque from said powertrainto first and second secondary wheels, said secondary driveline includinga rotary input member driven by said powertrain, a first rotary outputmember driving said first secondary wheel, a second rotary output memberdriving said second secondary wheel and a drive mechanism coupling saidinput member to said first and second output members, said drivemechanism including a differential assembly, a planetary gear assembly,first and second clutches and a brake, said differential assembly havingan input component driven by said input member, a first output componentdriving said first output member and a second output component drivingsaid second output member, said planetary gear assembly having first,second, third and fourth elements with said first element coupled tosaid first output member, said first clutch is operable for selectivelycoupling said second element of said planetary gear assembly to one ofsaid input component and said second output component of saiddifferential assembly, said second clutch is operable for selectivelycoupling said third element of said planetary gear assembly to the sameone of said input component and said second output component of saiddifferential assembly, and said brake is operable for selectivelybraking rotation of said fourth element of said planetary gear assembly;and a control system for controlling actuation of said first and secondclutches and said brake.
 2. The motor vehicle of claim 1 wherein saiddrive mechanism is operable to establish a first drive mode when saidbrake is engaged with said first clutch engaged and said second clutchreleased such that said first output member is overdriven relative tosaid second output member.
 3. The motor vehicle of claim 2 wherein saiddrive mechanism is operable to establish a second drive mode when saidbrake is engaged with said second clutch engaged and said first clutchreleased such that said first output member is underdriven relative tosaid second output member.
 4. The motor vehicle of claim 3 wherein saiddrive mechanism is operable to establish a third drive mode when saidbrake is engaged with both of said first and second clutches engagedsuch that speed differentiation is prevented between said first andsecond output members.
 5. The motor vehicle of claim 3 wherein saiddrive mechanism is operable to establish a third drive mode when saidbrake is released with both of said first and second clutches releasedsuch that uninhibited speed differentiation is permitted between saidfirst and second output members.
 6. The motor vehicle of claim 1 whereinsaid input component of said differential assembly is a differentialcarrier, said first output component is a first side gear coupled forrotation with said first output member, and said second output componentis a second side gear coupled for rotation with said second outputmember, and wherein said differential assembly further includes piniongears supported by said differential carrier and which are meshed withsaid first and second side gears.
 7. The motor vehicle of claim 1wherein said input component of said differential assembly is a ringgear, said first output component is a sun gear coupled for rotationwith said first output member, and said second output component is acarrier coupled for rotation with said second output member, and whereinsaid differential assembly further includes pinion gears supported bysaid carrier and which establish a meshed connection between said ringgear and said sun gear.
 8. The motor vehicle of claim 1 wherein saidplanetary gear assembly includes a first carrier fixed for rotation withsaid first output member and which rotatably supports first planetgears, a first sun gear meshed with said first planet gears, a firstring gear meshed with said first planet gears, a second sun gear, asecond ring gear coupled to said first carrier, and a second carrierfixed to said first ring gear and which rotatably supports second planetgears that are meshed with said second sun gear and said second ringgear.
 9. The motor vehicle of claim 8 wherein said first clutch isoperable to selectively couple said first sun gear to said inputcomponent of said differential assembly, wherein said second clutch isoperable to selectively couple said first ring gear to said inputcomponent of said differential assembly, and wherein said brake isoperable to selectively brake rotation of said second sun gear.
 10. Themotor vehicle of claim 8 wherein said first clutch is operable toselectively couple said first sun gear to said second output componentof said differential assembly, wherein said second clutch is operable toselectively couple said first ring gear to said second output componentof said differential assembly, and wherein said brake is selectivelyoperable to brake rotation of said second sun gear.
 11. A drive axleassembly for use in a motor vehicle having a powertrain and first andsecond wheels, comprising: an input shaft driven by the powertrain; afirst axleshaft driving the first wheel; a second axleshaft driving thesecond wheel; a differential assembly having an input component drivenby said input shaft, a first output component driving said firstaxleshaft and a second output component driving said second axleshaft; aplanetary gear assembly having first, second, third and fourth elementswith said first element coupled for rotation with said first axleshaft;a first clutch for selectively coupling said second element of saidplanetary gear assembly to one of said input component and said secondoutput component of said differential assembly; a second clutch forselectively coupling said third element of said planetary gear assemblyto one of said input component and said second output component of saiddifferential assembly; a brake for selectively braking rotation of saidfourth element of said planetary gear assembly; and a control system forcontrolling actuation of said first and second clutches and said brake.12. The drive axle assembly of claim 11 wherein a first drive mode isestablished when said brake is engaged with said first clutch engagedand said second clutch released such that said first axleshaft isoverdriven relative to said second axleshaft.
 13. The drive axleassembly of claim 12 wherein a second drive mode is established whensaid brake is engaged with said second clutch engaged and said firstclutch released such that said first axleshaft member is underdrivenrelative to said second axleshaft.
 14. The drive axle assembly of claim13 wherein a third drive mode is established when said brake is engagedwith both of said first and second clutches engaged such that speeddifferentiation is prevented between said first and second axleshafts.15. The drive axle assembly of claim 13 wherein a third drive mode isestablished when said brake is released with both of said first andsecond clutches released such that uninhibited speed differentiation ispermitted between said first and second axleshafts.
 16. The drive axleassembly of claim 11 wherein said input component of said differentialassembly is a differential carrier, said first output component is afirst side gear coupled for rotation with said first axleshaft, saidsecond output component is a second side gear coupled for rotation withsaid second axleshaft, and wherein said differential assembly furtherincludes pinion gears supported by said differential carrier and meshedwith said first and second side gears.
 17. The drive axle assembly ofclaim 11 wherein said input component of said differential assembly is aring gear, said first output component is a sun gear coupled forrotation with said first axleshaft, said second output component is acarrier coupled for rotation with said second axleshaft, and whereinsaid differential assembly further includes pinion gear supported bysaid carrier and establishing a meshed connection between said ring gearand said sun gear.
 18. The drive axle assembly of claim 11 wherein saidplanetary gear assembly includes a first carrier fixed for rotation withsaid first axleshaft and which rotatably supports first planet gears, afirst sun gear meshed with said first planet gears, a first ring gearmeshed with said first planet gears, a second sun gear, a second ringgear coupled to said first carrier, and a second carrier fixed to saidfirst ring gear and which rotatably supports second planet gears thatare meshed with said second sun gear and said second ring gear.
 19. Thedrive axle assembly of claim 18 where in said first clutch is operableto selectively couple said first sun gear to said input component ofsaid differential assembly, wherein said second clutch is operable toselectively couple said first ring gear to said input component of saiddifferential assembly, and wherein said brake is operable to selectivelybrake rotation of said second sun gear.
 20. The drive axle assembly ofclaim 18 wherein said first clutch is operable to selectively couplesaid first sun gear to said second output component of said differentialassembly, wherein said second clutch is operable to selectively couplesaid first ring gear to said second output component of saiddifferential assembly, and wherein said brake is selectively operable tobrake rotation of said second sun gear.
 21. A drive axle assembly foruse in a motor vehicle having a powertrain and first and second wheels,comprising: an input shaft driven by the powertrain; a first axleshaftdriving the first wheel; a second axleshaft driving the second wheel; adifferential assembly having an input component driven by said inputshaft, a first output component coupled for rotation with said firstaxleshaft and a second output component coupled for rotation with saidsecond axleshaft; a first gearset having a first sun gear, a first ringgear, and a first planet carrier coupled for rotation with said firstaxleshaft and which rotatably supports first planet gears that aremeshed with said first sun gear and said first ring gear; a secondgearset having a second sun gear, a second ring gear fixed for rotationwith said first planet carrier, and a second planet carrier coupled forrotation with said first ring gear and which rotatably supports secondplanet gears that are meshed with said second sun gear and said secondring gear; a brake for selectively inhibiting rotation of said secondsun gear; a first clutch for selectively coupling said first sun gear toone of said input component and said second output component of saiddifferential assembly; a second clutch for selectively coupling saidfirst ring gear to one of said input component and said second outputcomponent of said differential assembly; and a control system forcontrolling actuation of said first and second clutches and said brake.22. The drive axle assembly of claim 21 wherein said first clutch isoperable in a first mode to permit rotation of said first sun gearrelative to said input component and in a second mode to prevent suchrelative rotation therebetween, wherein said second clutch is operablein a first mode to permit rotation of said first ring gear relative tosaid input component and in a second mode to prevent such relativerotation therebetween, wherein said brake is operable in a first mode topermit rotation of said second sun gear and in a second mode to preventrotation of said second sun gear, and wherein said control system isoperable for shifting each of said first and second clutches and saidbrake between their corresponding first and second modes.
 23. The driveaxle assembly of claim 22 wherein a first mode is established when saidbrake is in its second mode with said first clutch in its second modeand said second clutch in its first mode such that said first axleshaftis driven at an increased rotary speed relative to said secondaxleshaft, and wherein a second drive mode is established when saidbrake is in its second mode with said first clutch in its first mode andsaid second clutch in its second mode such that said first axleshaft isdriven at a reduced rotary speed relative to said second axleshaft. 24.A transfer case for a four-wheel drive vehicle having a powertrain andfirst and second drivelines, comprising: an input shaft driven by thepowertrain; a first output shaft driving the first driveline; a secondoutput shaft driving the second driveline; a torque transfer mechanismoperably interconnecting said input shaft to said first and secondoutput shafts, said torque transfer mechanism including a differentialassembly, a planetary gear assembly, a brake and first and secondclutches, said differential assembly having an input component driven bysaid input shaft, a first output component driving said first outputshaft and a second output component driving said second output shaft,said planetary gear assembly having first, second, third and fourthelements with said first element coupled to said first output shaft,said first clutch is operable for selectively coupling said secondelement of said planetary gear assembly to one of said input componentand said second output component of said differential assembly, saidsecond clutch is operable for selectively coupling said third element ofsaid planetary gear assembly to the same one of said input component andsaid second output component of said differential assembly, and saidbrake is operable for selectively braking rotation of said fourthelement of said planetary gear assembly; and a control system forcontrolling actuation of said first and second clutches and said brake.25. A power transfer assembly for use in a motor vehicle having apowertrain and first and second drivelines, comprising: an input shaftdriven by the powertrain; a first shaft driving the first driveline; asecond shaft driving the second driveline; a differential assemblyhaving an input component driven by said input shaft, a first outputcomponent fixed for rotation with said first shaft, and a second outputcomponent fixed for rotation with said second shaft; a planetary gearassembly having first, second, third and fourth elements with said firstelement coupled to said first shaft; a first clutch operable forselectively coupling said second element of said planetary gear assemblyto one of said input component and said second output component of saiddifferential assembly; a second clutch operable for selectively couplingsaid third element of said planetary gear assembly to one of said inputcomponent and said second output component of said differentialassembly; a brake operable for selectively braking rotation of saidfourth element of said planetary gear assembly; and a control system forcontrolling actuation of said brake and said first and second clutches.